Exposure to microgravity causes short- and long-term changes in sensorimotor integration as indicated by astronauts' clinically significant decrements in performance of tasks that require sensorimotor integration shortly after landing. These deficits have potentially serious consequences for crew safety at landing and for crew efficiency in the first few days after landing, either on Earth or some other planetary surface. Likewise, patients with vestibularly-related balance impairments have functional limitations in tasks requiring good dynamic balance and visual acuity, with significant implications for their safety and for their quality of life. Thus countermeasures, i.e., rehabilitative treatments, are needed for both populations, to facilitate acquisition or re-acquisition of adaptive motor strategies in response to sensorimotor challenges. This study will test a rehabilitation approach for maintaining or rapidly reaquiring terrestrial strategies for dynamic balance and sensorimotor integration after exposure to microgravity and for acquiring new movement strategies after loss or impairment of vestibular function. The study will have the following specific aims: 1) Determine the effectiveness of sensorimotor practice variability for transfer of the ability to "learn to learn" adaptive sensorimotor strategies under different conditions of visual/vestibular rearrangement. We will also determine the changes in head-trunk coordination and lower limb kinematics during gait and determine which kinematic patterns are most compatible with successful motor performance. We hypothesize that subjects trained with variable practice on either visual rearrangement (induced with three sets of experimental lenses) or exercise (with treadmill walking at two speeds) will have significantly better scores than subjects trained in the constant practice groups when post-tested on transfer trials using a fourth set of lenses and a third speed. We further hypothesize that subjects trained with variable practice on both visual rearrangement and exercise will have significantly better scores than subjects in constant practice groups and subjects in groups given variable practice on only one parameter. 2) Determine the effectiveness of sensorimotor practice variability for retention of the ability to "learn to learn" adaptive sensorimotor strategies under different conditions of visual/ vestibular rearrangement. We hypothesize that on retention trials given one month after the post-test transfer trials in Specific Aim 1, subjects trained with variable practice on either visual rearrangement or exercise will have significantly better scores than subjects trained in the constant practice groups; furthermore, subjects trained on variable practice on both parameters will have significantly better scores than subjects in constant practice groups and subjects in groups given variable practice on only one parameter.